The present invention relates to a portamento system and more particularly to a variable rate three mode portamento system for use with electronic musical instruments. The disclosure is facilitated by describing the portamento system in conjunction with a keyboard instrument which utilizes master oscillator/divider elements for tone generation.
Portamento is a musical term defined as the passage from one tone to another in a continuous glide or progression through the intervening tones.
In keyboard instruments which utilize a master oscillator/divider, tones are generated only at discrete frequency intervals. Therefore, in order to obtain the continuous glide or progression through frequencies required for a portamento effect, a separate system having its own oscillator must be provided. In order for the portamento effect to be effectively utilized, it must be controlled from the keyboard of the electronic musical instrument with which it is being used. Thus, a voltage controlled oscillator (VCO) deriving its control voltage directly from the keyboard of the electronic musical instrument or from a frequency to voltage converter connected thereto could be used to provide the continuous glide or tone progression characteristic of the portamento affect. However, the frequency output of such a system will be related only indirectly to the main tone generator system of the electronic musical instrument, and therefore some amount of frequency error will always be present.
One approach to eliminating the frequency error as disclosed herein is to use a phase-locked loop integrated circuit in conjunction with a free running voltage controlled oscillator (VCO) to lock up the frequency output thereof with a desired input frequency from the electronic musical instrument with a minimal amount of phase error. However, phase-locked loop integrated circuits operate over only a relatively limited frequency range when used without external drive circuitry and do not have any means of controlling the lock up rate, which rate corresponds to the rate at which the portamento effect glides or progresses between its beginning and ending tones. Thus, previous portamento systems have generally been limited to only one sine wave footage or voice representing a relatively limited frequency range from a keyboard instrument, and have further generally been limited in producing satisfactory glide rates.
Thus, it would be desirable to provide a portamento system which can be utilized with a plurality of voices of an electronic musical instrument, having a relatively broad frequency range and satisfactorily operable at a wide range of speeds which may be selectively chosen to give speeds as fast or as slow as desired to meet the requirements of specific musical applications.
Also, previous portamento systems have generally been limited to two modes of operation, namely, "on" or "off". It is desirable in this regard to provide a portamento system having additional modes of operation to meet the requirements of a variety of musical applications.